tests: fix nondeterministic test failure in test-contrib-perf.t

It turns out (not too shockingly!) the kernel sometimes has some work to do,
perhaps at the very least context-switching, so asserting the system time
is 0.000000 doesn't work.

tests/test-contrib-perf.t

@@ -309,9 +309,9 b' Multiple entries'
 
   $ hg perfparents --config perf.stub=no --config perf.run-limits='500000-1, 0.000000001-50'
   ! wall * comb * user * sys * (best of 50) (glob)
-  ! wall * comb * user * sys 0.000000 (max of 50) (glob)
-  ! wall * comb * user * sys 0.000000 (avg of 50) (glob)
-  ! wall * comb * user * sys 0.000000 (median of 50) (glob)
+  ! wall * comb * user * sys * (max of 50) (glob)
+  ! wall * comb * user * sys * (avg of 50) (glob)
+  ! wall * comb * user * sys * (median of 50) (glob)
 
 error case are ignored